Measurements of the branching fraction ratio $\mathcal{B}(\phi\to\mu^{+}\mu^{-})$/$\mathcal{B}(\phi\to e^{+}e^{-})$ with charm meson decays

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LHCb-PAPER-2023-038
CERN-EP-2024-001
arXiv:2402.01336 [PDF]

(Submitted on 02 Feb 2024)
JHEP
Inspire 2755088

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Abstract

Measurements of the branching fraction ratio ${\cal{B}(\phi \to \mu^+ \mu^-)/\cal{B}(\phi\to e^+e^-)}$ with ${D_{s}^{+} \to \pi^{+} \phi}$ and ${D^{+} \to \pi^{+} \phi}$ decays, denoted $R^{s}_{\phi \pi}$ and $R^{d}_{\phi \pi}$, are presented. The analysis is performed using a dataset corresponding to an integrated luminosity of 5.4$ \rm{fb}^{-1}$ of $pp$ collision data collected with the LHCb experiment. The branching fractions are normalised with respect to the ${B^{+} \to K^{+} J/\psi(\to e^+e^-)}$ and ${B^{+} \to K^{+} J/\psi(\to \mu^+\mu^-)}$ decay modes. The combination of the results yields $$ R_{\phi \pi} = 1.022 \pm 0.012 ({\rm stat}) \pm 0.048 ({\rm syst}). $$ The result is compatible with previous measurements of the $\phi \to \ell^{+}\ell^{-}$ branching fractions and predictions based on the Standard Model.

Figures and captions

Distribution of the invariant mass $ m_{\phi}{(\pi ^+ \ell^+ \ell^- )}$ overlaid with the result of the fit for candidates with (left) muon and (right) electron pairs in the final states. The dotted and dot-dashed lines describe the signal contributions, while the solid areas represent the background components.	Fig1a.pdf [25 KiB] HiDef png [215 KiB] Thumbnail [167 KiB] *.C file
	Fig1b.pdf [26 KiB] HiDef png [289 KiB] Thumbnail [228 KiB] *.C file
Left: distribution of the $\pi ^+ \ell^{+}\ell^{-}$ invariant mass of candidates obtained from selecting final states with electrons of same charge (blue) or that fail the BDT requirement (orange). Fit results using a third-order polynomial are overlaid. The overall normalisation is chosen such that the two curves have the same area. Right: distribution of the invariant mass of $\pi^{+}\ell^{-}\ell^{+}$ candidates, after interpreting the leptons as pions, for candidates failing the electron PID criteria. The reversed PID sample contains predominantly $ D^{+}_{(s)}\rightarrow \pi ^+ \pi ^- \pi ^+ $ events and a fraction of combinatorial events.	Fig2a.pdf [27 KiB] HiDef png [393 KiB] Thumbnail [244 KiB] *.C file
	Fig2b.pdf [24 KiB] HiDef png [273 KiB] Thumbnail [195 KiB] *.C file
Invariant mass distributions of the selected candidates for the (left) $ { B ^+ \rightarrow K^{+} { J \mskip -3mu/\mskip -2mu\psi } (\rightarrow e ^+ e ^- )}$ and (right) $ { B ^+ \rightarrow K^{+} { J \mskip -3mu/\mskip -2mu\psi } (\rightarrow \mu ^+\mu ^- )}$ modes, overlaid with the result of the fit. The dotted lines describe the signal contribution and the solid areas represent each of the background components described in the text.	Fig3a.pdf [41 KiB] HiDef png [235 KiB] Thumbnail [197 KiB] *.C file
	Fig3b.pdf [25 KiB] HiDef png [256 KiB] Thumbnail [204 KiB] *.C file
Distributions of (top) $m(e^{+}e^{-})$ and (bottom) $m(\mu^{+}\mu^{-})$ for candidates selected in the signal region of the (left) $ D ^+$ (right) and $ D ^+_ s $ $\phi$-constrained invariant mass. The dotted lines describe the signal contribution and the solid areas represent each of the background components described in the text.	Fig4a.pdf [24 KiB] HiDef png [248 KiB] Thumbnail [204 KiB] *.C file
	Fig4b.pdf [25 KiB] HiDef png [236 KiB] Thumbnail [189 KiB] *.C file
	Fig4c.pdf [23 KiB] HiDef png [184 KiB] Thumbnail [154 KiB] *.C file
	Fig4d.pdf [24 KiB] HiDef png [173 KiB] Thumbnail [146 KiB] *.C file
Distribution of the invariant mass computed under the pion mass hypothesis for the dielectron pair using track information only, for candidates passing the electron PID criteria. The fit projection is superimposed, with dashed and dotted-dashed lines describing the signal contributions, while solid light and dark blue areas represent misidentified $ D^{+}_{(s)}\rightarrow \pi ^+ \pi ^- \pi ^+ $ components. In this variable the latter are clearly visible in data passing the electron PID criteria.	Fig5.pdf [27 KiB] HiDef png [306 KiB] Thumbnail [247 KiB] *.C file
Measured $ R^{(d,s)}_{\phi \pi}$ as a function of the (top left) angle between the leptons $\alpha(\ell^+, \ell^-)$, and (top right) the maximum $ p_{\mathrm{T}}$ of the leptons. The structure observed at intermediate maximum $ p_{\mathrm{T}}$ is related to the trigger threshold. The errors shown are statistical only. Also shown is the ratio of efficiency-corrected ${ D^{+}_{(s)} \rightarrow \pi ^+ \phi }$ and ${ { B ^+ \rightarrow K^{+} { J \mskip -3mu/\mskip -2mu\psi } (\rightarrow e ^+ e ^- )} }$ yields as a function of the (bottom left) transverse momentum recovered with the bremsstrahlung recovery algorithm and (bottom right) its fraction with respect to the total transverse momentum of the electron. The flatness of these distributions indicates that the bremsstrahlung recovery algorithm is well reproduced in simulation at low $ q^2$ . The normalisation of these distributions is arbitrary and the uncertainties displayed are statistical only.	Fig6a.pdf [99 KiB] HiDef png [144 KiB] Thumbnail [111 KiB] *.C file
	Fig6b.pdf [99 KiB] HiDef png [158 KiB] Thumbnail [121 KiB] *.C file
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Animated gif made out of all figures.	PAPER-2023-038.gif Thumbnail

Tables and captions

Signal ( $ D^{+}_{(s)} \rightarrow \pi ^+ \phi$ ) and normalisation ( $ { B ^+ \rightarrow K^{+} { J \mskip -3mu/\mskip -2mu\psi } (\rightarrow \ell^{+}\ell^{-})}$ ) mode sideband mass ranges. The variables $ m_{\phi}{(\pi ^+ \ell^+ \ell^- )}$ and $ m_{ { J \mskip -3mu/\mskip -2mu\psi } }{( K ^+ \ell^+ \ell^- )}$ are used for signal and normalisation decays, respectively.	Table_1.pdf [68 KiB] HiDef png [46 KiB] Thumbnail [21 KiB] tex code
Yields of the signal and normalisation decay modes obtained from the fits to the data. The quoted uncertainty accounts for both statistical and systematic effects.	Table_2.pdf [68 KiB] HiDef png [92 KiB] Thumbnail [44 KiB] tex code
Relative systematic uncertainties in percent affecting the measurements $ R^{d}_{\phi \pi}$ and $ R^{s}_{\phi \pi}$ .	Table_3.pdf [58 KiB] HiDef png [69 KiB] Thumbnail [31 KiB] tex code

Supplementary Material [file]

Supplementary material full pdf	supple[..].pdf [743 KiB]
This ZIP file contains supplemetary material for the publication LHCb-PAPER-2023-038. The files are: supplementary.pdf : An overview of the extra figures supplementary.tex : The corresponding TeX source code .pdf, .png, *.eps : The figures in various formats	Fig6a.pdf [13 KiB] HiDef png [121 KiB] Thumbnail [107 KiB] *C file
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